LNT desulfation strategy with reformer temperature management

ABSTRACT

Desulfation methods for an exhaust treatment system having a fuel reformer configured upstream of a LNT. Reductant is injected upstream of the fuel reformer. The reductant reacts within the reformer to generate heat, but the system is configured for some reductant to breakthrough and react in the LNT to generate further heat. This configuration allows the LNT to operate at temperatures higher the than first device and facilitates independent control of the LNT and first device temperatures.

PRIORITY

This application is a divisional of U.S. application Ser. No.11/293,065, filed Dec. 2, 2005.

FIELD OF THE INVENTION

The present invention relates to pollution control systems and methodsfor diesel and lean burn gasoline engines.

BACKGROUND

NO_(x) emissions from diesel engines are an environmental problem.Several countries, including the United States, have long hadregulations pending that will limit NO_(x) emissions from trucks andother diesel-powered vehicles. Manufacturers and researchers have putconsiderable effort toward meeting those regulations.

In gasoline powered vehicles that use stoichiometric fuel-air mixtures,three-way catalysts have been shown to control NO_(x) emissions. Indiesel-powered vehicles, which use compression ignition, the exhaust isgenerally too oxygen-rich for three-way catalysts to be effective.

Several solutions have been proposed for controlling NOx emissions fromdiesel-powered vehicles. One set of approaches focuses on the engine.Techniques such as exhaust gas recirculation and partially homogenizingfuel-air mixtures are helpful, but these techniques alone will noteliminate NOx emissions. Another set of approaches remove NOx from thevehicle exhaust. These include the use of lean-burn NO_(X) catalysts,selective catalytic reduction (SCR), and lean NO_(X) traps (LNTs).

Lean-burn NOx catalysts promote the reduction of NO_(x) underoxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere isdifficult. It has proven challenging to find a lean-burn NO_(x) catalystthat has the required activity, durability, and operating temperaturerange. Lean-burn NO_(x) catalysts also tend to be hydrothermallyunstable. A noticeable loss of activity occurs after relatively littleuse. Lean-burn NOx catalysts typically employ a zeolite wash coat, whichis thought to provide a reducing microenvironment. The introduction of areductant, such as diesel fuel, into the exhaust is generally requiredand introduces a fuel economy penalty of 3% or more. Currently, peak NOxconversion efficiencies for lean-burn catalysts are unacceptably low.

SCR generally refers to selective catalytic reduction of NOx by ammonia.The reaction takes place even in an oxidizing environment. The NOx canbe temporarily stored in an absorbent or ammonia can be fed continuouslyinto the exhaust. SCR can achieve high levels of NOx reduction, butthere is a disadvantage in the lack of infrastructure for distributingammonia or a suitable precursor. Another concern relates to the possiblerelease of ammonia into the environment.

LNTs are devices with NOx absorbents and catalysts that reduce NOxduring regeneration. The absorbent is typically an alkaline earth oxideabsorbent, such as BaCO₃ and the catalyst is typically a precious metal,such as Pt or Ru. In lean exhaust, the catalyst speeds oxidizingreactions that lead to NOx adsorption. Accumulated NOx is removed andthe LNT is regenerated by creating a reducing environment within theLNT. In a reducing environment, the catalyst activates reactions bywhich adsorbed NOx is reduced and desorbed.

A LNT can produce ammonia during denitration. Accordingly, it has beenproposed to combine a LNT and an ammonia SCR catalyst into one system.Ammonia produced by the LNT during regeneration is captured by the SCRcatalyst for subsequent use in reducing NOx, thereby improvingconversion efficiency over a stand-alone LNT with no increase in fuelpenalty or precious metal usage. U.S. Pat. No. 6,732,507 describes sucha system. U.S. Pat. Pub. No. 2004/0076565 describes such systems whereinboth components are contained within a single shell or disbursed overone substrate. WO 2004/090296 describes such a system wherein there isan inline reformer upstream of the LNT and the SCR catalyst.

Creating a reducing environment for LNT regeneration involveseliminating most of the oxygen from the exhaust and providing a reducingagent. Except where the engine can be run stoichiometric or rich, aportion of the reductant reacts within the exhaust to consume oxygen.The amount of oxygen to be removed by reaction with reductant can bereduced in various ways. If the engine is equipped with an intake airthrottle, the throttle can be used. The transmission gear ratio can bechanged to shift the engine to an operating point that produces equalpower but contains less oxygen. However, at least in the case of adiesel engine, it is generally necessary to eliminate some of the oxygenin the exhaust by combustion or reforming reactions with reductant thatis injected into the exhaust.

Reductant can be injected into the exhaust by the engine or a separatefuel injection device. For example, the engine can inject extra fuelinto the exhaust within one or more cylinders prior to expelling theexhaust. Alternatively, or in addition, reductant can be injected intothe exhaust downstream of the engine.

The reactions between reductant and oxygen can take place in the LNT,although it is generally preferred for the reactions to occur in acatalyst upstream of the LNT, whereby the heat of reaction does notcause large temperature increase within the LNT at every regeneration.

In addition to accumulating NOx, LNTs accumulate SOx. SOx is thecombustion product of sulfur present in ordinarily fuel. Even withreduced sulfur fuels, the amount of SOx produced by combustion issignificant. SOx adsorbs more strongly than NOx and necessitates a morestringent, though less frequent, regeneration. Desulfation requireselevated temperatures as well as a reducing atmosphere. The temperatureof the exhaust can be elevated by engine measures, particularly in thecase of a lean-burn gasoline engine, however, at least in the case of adiesel engine, it is often necessary to provide additional heat.Typically, this heat is provided through the same types of reactions asused to remove excess oxygen from the exhaust.

U.S. Pat. No. 6,832,473 describes a system wherein the reductant isreformate produced outside the exhaust stream and injected into theexhaust as needed. During desulfations, the reformate is injectedupstream of an oxidation catalyst. Heat generated by combustion of thereformate over the oxidation catalyst is carried by the exhaust to theLNT and raises the LNT to desulfations temperatures.

U.S. Pat. Pub. No. 2003/0101713 describes an exhaust treatment systemwith a fuel reformer placed in the exhaust line upstream of a LNT. Thereformer includes both oxidation and reforming catalysts. The reformerboth removes excess oxygen and converts the diesel fuel reductant intomore reactive reformate. For desulfations, heat produced by the reformeris used to raise the LNT to desulfations temperatures. The diesel fuelinjection may be pulsed to control the reformer temperature.

In spite of advances, there continues to be a long felt need for anaffordable and reliable exhaust treatment system that is durable, has amanageable operating cost (including fuel penalty), and is practical forreducing NOx emissions from diesel engines to a satisfactory extent inthe sense of meeting U.S. Environmental Protection Agency (EPA)regulations effective in 2010 and other such regulations.

SUMMARY

Several of the inventors' concepts relates to methods of desulfating aLNT. The methods generally apply to a system in which a first device,which may be a fuel reformer, is placed upstream of the LNT in anexhaust treatment system. Reductant is injected upstream of the firstdevice. Much of the reductant is oxidized and/or reformed in the firstdevice and generates heat there.

According to one of the inventors' concepts for desulfating a LNT, asignificant additional portion of the reductant and/or reformed productsthereof react within the LNT to generate further heat. Typically, theLNT temperature is thereby raised to a peak that is at least about 100°C. higher than the temperature of the exhaust entering the LNT. Thisconcept allows the LNT to operate at higher temperatures than the firstdevice and facilitates independent control of the LNT and first devicetemperatures in conjunction with methods described below.

Another concept relates to a control strategy that applies to theabove-described system. An outer loop controls the LNT temperature byissuing instructions to an inner loop that controls the first device.The instructions involve, for example, switching modes for the innerloop (e.g., between on and off modes). The inner loop controls the firstdevice temperature through control at least over the reductant injectionrate. Typically, the inner loop will pulse the reductant injection ratein order to limit the first device temperature. The outer loop can pulsethe operation of the inner loop to independently control the temperatureof the LNT.

A similar concept also relates to a method of desulfating a LNT in anexhaust aftertreatment system comprising a first device upstream of theLNT. The method comprises controlling a temperature of the LNT to withina first range by varying a supply of diesel fuel to the exhaust upstreamof the first device. The control strategy comprising two modes of dieselfuel supply, a first mode of which comprises supplying diesel fuel in amanner that regulates the temperature of the first catalyst to within asecond range having a minimum and a maximum, and a second mode in whichthe diesel fuel supply is substantially terminated, whereby thetemperature of the first device falls below the minimum of the secondrange.

A further concept relates to a control strategy for desulfation thatinvolves reductant injection that is intermittent on two time scales,one shorter and one longer. The intermittency on the longer time scaleis generally used to control the temperature of an LNT. Theintermittency on the shorter time scale is generally used to control thetemperature of an upstream device, the first device referred to above,which is typically a fuel reformer.

A still further concept also relates to a desulfation control strategyfor an exhaust treatment system having a first device upstream of a LNTand a reductant supply upstream of the first device. According to thisstrategy, the reductant supply is pulsed to control the temperature ofthe LNT. The pulse periods and durations are selected whereby reductantis provided to the LNT while the LNT temperature is still near a peakresulting from combustion of reductant injected during the previouspulse. A portion of the reductant, pyrolyzed, or reformed productsthereof, maybe stored in the LNT during periods or reductant injectionand may combust during periods of no reductant injection when the oxygenconcentration within the LNT increases. If the LNT temperature variesover each pulse period between a minimum and a maximum differing by atleast about 50° C., the LNT temperature will generally be within about25° C. of the maximum at the onset of reductant injection.

The inventors' concepts include power generation systems and vehiclesconfigured to implement one of the methods described herein.

The primary purpose of this summary has been to present certain of theinventors' concepts in a simplified form to facilitate understanding ofthe more detailed description that follows. This summary is not acomprehensive description of every one of the inventors' concepts orevery combination of the inventors concepts that can be considered“invention”. Other concepts of the inventors will be conveyed to one ofordinary skill in the art by the following detailed description togetherwith the drawings. The specifics disclosed herein may be generalized,narrowed, and combined in various ways with the ultimate statement ofwhat the inventors claim as their invention being reserved for theclaims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary exhaust treatmentsystem in which several concepts of the inventors can be implemented.

FIG. 2 is a schematic illustration of another exemplary exhausttreatment system in which several concepts of the inventors can beimplemented.

FIG. 3 is a schematic of a control architecture exemplifying one of theinventors' concepts.

FIG. 4 is a plot showing an exemplary pattern of fuel injection andresulting reformer and LNT temperature profiles.

FIG. 5 is a plot showing exemplary temperatures and reformateconcentrations in a LNT downstream of a reformer when fuel is suppliedas shown in FIG. 4.

FIG. 6 is a plot showing the relationship between rates of desulfationand conditions plotted in FIG. 5.

FIG. 7 is a flow chart of an exemplary control process that can be usedin implementing several of the inventors' concepts.

DETAILED DESCRIPTION

Various concepts of the inventors are applicable to exhaust treatmentsystems having a first device upstream of a LNT. The first devicecomprises a catalyst that is operative to consume a substantial portionof the oxygen contained in the exhaust by reaction with a reductantadded to the exhaust. A measurable amount of oxygen will always bepresent in the exhaust downstream of such a device, but in general thedevice will be adapted to remove most of the oxygen from the exhaust.

Preferably the first device is operative to produce reformate from afossil fuel reductant, such as diesel fuel, but the concepts of theinventors are not so limited. For example, the various concepts can beapplied to a system in which reformate is produced outside the exhaustline and is injected upstream of a first device, which operates simplyas an oxidation catalyst.

FIG. 1 provides a schematic illustration of an exemplary powergeneration system 5 in which various concepts of the inventors can beimplemented. The system 5 comprises an engine 9, a transmission 8, andan exhaust aftertreatment system 7. The exhaust aftertreatment system 7includes a controller 10, a fuel injector 11, a lean NOx catalyst 15, areformer 12, a lean NOx-trap (LNT) 13, an ammonia-SCR catalyst 14, adiesel particulate filter (DPF) 16, and a clean-up catalyst 17. Thecontroller 10 receives data from several sources; include temperaturesensors 20 and 21 and NOx sensors 22 and 23. The controller 10 may be anengine control unit (ECU) that also controls the transmission 8 and theexhaust aftertreatment system 7 or may include several control unitsthat collectively perform these functions.

The transmission 8 can be of any type. In some embodiments, however, thetransmission 8 is of a type that allows selection from among a largenumber of widely ranging torque multipliers and makes available a rangeof operating points at which the engine 9 can meet a given power demand.For example, the transmission 8 can be a continuously variabletransmission (CVT).

The lean-NOx catalyst 15 removes a portion of the NOx from the engineexhaust using reductants, typically hydrocarbons that form part of theexhaust or have been stored by the lean-NOx catalyst 15. The DPF 16removes particulates from the exhaust. During lean operation (a leanphase), the LNT 13 adsorbs a second portion of the NOx. The ammonia-SCRcatalyst 14 may have ammonia stored from a previous regeneration of theLNT 13 (a rich phase). If the ammonia-SCR catalyst 14 contains storedammonia, it removes a third portion of the NOx from the lean exhaust.The clean-up catalyst 17 may serve to oxidize CO and unburnedhydrocarbons remaining in the exhaust.

From time-to-time, the LNT 13 must be regenerated to remove accumulatedNOx (denitrated). Denitration may involve heating the reformer 12 to anoperational temperature and then injecting fuel using the fuel injector11. The reformer 12 uses the injected fuel to consume most of the oxygenfrom the exhaust while producing reformate. The reformate thus producedreduces NOx adsorbed in the LNT 13. Some of this NOx is reduced to NH₃,most of which is captured by the ammonia-SCR catalyst 14 and used toreduce NOx during a subsequent lean phase. The clean-up catalyst 17oxidizes unused reductants and unadsorbed NH₃ using stored oxygen orresidual oxygen remaining in the exhaust during the rich phases. Duringregeneration, the lean-NOx catalyst 15 may store reductant for lateruse.

FIG. 2 provides another exemplary system 25 to which various concepts ofthe invention can be applied. The system 25 contains many of the samecomponents as the system 5, although it does not include the lean NOxcatalyst 15 or the cleanup oxidation catalyst 17. One significantdifference is that in the system 25 the DPF 14 is placed between thereformer 12 and the LNT 13. This configuration may facilitate timing theLNT temperature peaks to nearly coincide with the periods of reductantinjection, as explained further below. The DPF 14 may also serve toprotect the LNT 13 from high temperatures during denitrations byproviding a thermal buffer between the reformer 12 and the LNT 13.Reducing the number and/or magnitude of temperature excursions in theLNT 14 may extend the life of the LNT 14.

From time-to-time, the LNT 13 must also be regenerated to removeaccumulated sulfur compounds (desulfated). Desulfation may involveheating the reformer 12 to an operational temperature, heating the LNT13 to a desulfating temperature, and providing the heated LNT 13 with areducing atmosphere. An operational temperature for the reformer dependson the reformer design. Desulfating temperatures also vary, but aretypically in the range from about 500 to about 800° C., more typicallyin the range from about 650 to about 750° C. Below a minimumtemperature, desulfation is very slow. Above a maximum temperature, theLNT 13 may be damaged.

One concept relates to a method of achieving or maintaining desulfationtemperatures in the LNT 13. According to this method the LNT 13 isheated by measures that include injecting reductant into the exhaust. Aportion of the reductant reacts in the reformer 12, or an oxidationcatalyst, to generate heat for raising the temperature of the LNT 13,but an additional portion of the reductant, or derivatives thereof,passes through the reformer 12 or oxidation catalyst and react in theLNT 13. The reactions in the LNT 13 generate a significant amount ofheat there. One advantage of this method is that the temperatures of theLNT 13 and the reformer 12 can be independently controlled. Anotheradvantage is that the reformer 12 does not have to be heated to thedegree required if the LNT 13 is heated purely by convective heattransfer from the reformer 12. This latter advantage may be particularlysignificant if there is a substantial heat loss to the surroundingsbetween the reformer 12 and the LNT 13. The latter advantage may also besignificant if there is an optimal temperature for operating thereformer 12 that is not conveniently related to the optimal temperaturefor desulfating the LNT 13.

The reductant supply to the reformer 12 is typically intermittent. Thiscan facilitate providing reagents for combustion in the LNT 13. Forexample, during a reductant injection phase, the exhaust is generallymade rich and oxygen concentration may be brought very low as theexhaust passes through the reformer 12. Nevertheless, there may beoxidation in the LNT 13 using oxygen stored there in a previous phase ofno reductant injection or using oxygen that accumulates in the spacebetween the reformer 12 and the LNT 13 if that space is sufficientlylarge. Another possibility is that the oxygen is oxygen slipping pastthe reformer 12 during the reductant injection phase. Such slippage maybe facilitated by low temperature operation of the reformer 12, limitedcatalyst loading, or limited mass transfer efficiency between theexhaust and that catalyst.

It is also possible that combustion within the LNT 13 takes place duringphase of no reductant injection when oxygen concentration in the exhaustis high. Such combustion can be realized if reductant, such ashydrocarbon, pyrolyzed reductant, or reformed reductant is adsorbed inthe LNT 13 during the reductant injection phases. Another possibility isthat one of these species accumulates in the space between the reformer12 and the LNT 13 during a reductant injection phase and mixes withoxygen during a subsequent phase of no reductant injection. In oneembodiment, the manner of supplying fuel to the LNT 13 for combustion inthe LNT 13 is to allow a diesel fuel reductant or pyrolyzed compoundsderived from that reductant (cracked diesel fuel) to slip past the firstdevice. Slip can be controlled through the activity and mass transfercharacteristic of the first device, the diesel fuel injection rate, andor the temperature to which the reformer 12 is controlled. The LNT 13generally contains a catalyst functional with respect to combustion ofmost reductants.

One measure of the extent to which the LNT 13 is heated by reactionsoccurring therein is a comparison between the peak temperature of theexhaust entering the LNT 13 during a desulfation process and the peaktemperature realized within the LNT 13 during that process. Preferably,through any suitable combination of measures, reactions are broughtabout in the LNT 13 that are sufficient to raise a temperature withinthe LNT 13 to a peak at least about 50° C. higher than the peaktemperature of the exhaust entering the LNT 13. More preferably thedifference is at least about 100° C. and still more preferably at leastabout 150° C.

A related concept is a control system comprising inner and outer loops.The outer loop controls the LNT temperature by issuing instructions toan inner loop. The inner loop controls the first device temperature(typically a reformer) through control over the reductant injection rateand possibly other measures as well. Other measures could include, forexample, an intake air throttle for the engine 9 or a torque ratioselection for the transmission 8. One advantage of this configuration isthat the inner loop generally has faster dynamic than the outer loop andcan rapidly respond to disturbances that affect the exhaust flow rate,temperature, or composition.

FIG. 3 provides a schematic of an exemplary control architecture 100illustrating inner and outer loop controls. The LNT temperaturecontroller 102 is activated by a desulfation scheduler/controller 101that applies any appropriate criteria to determine to when initiate adesulfation process. The LNT temperature controller 102 considers a LNTtemperature provided by a state estimator 103. It is preferred to use anobserver or state estimator to determine the LNT temperature, becausethe LNT temperature responds comparatively slowly to controllableparameters. If some form of prediction is not used, there is a risk ofthe LNT temperature exceeding an intended limit. An extrapolation basedon the current measured temperature, its rate of change, and an estimateof the temperature measurement delay is generally sufficient.

The output of the LNT temperature controller is instructions for thereformer temperature controller 106. The instructions may simply beinstructions for the reformer 12 to switch between active and inactivemodes. During the active mode, the reformer 12 is controlled to atemperature suitable for reformate production. During an inactive mode,the reformer 12 is generally “off”, meaning there is no reductantinjection and the reformer 12 is allowed to cool freely.

When the reformer 12 is to be active, the reformer temperaturecontroller 106 regulates the reformer temperature at least by issuingcommands to the injection controller 107. There are various types ofreformers, as described below, and various configurations within eachtype.

During rich operation of the reformer 12, the majority of the oxygenpresent in the exhaust is consumed while producing reformate. Regardlessof the actual sequence of reactions, the operation of the reformer 12can be modeled by a combination of reactions similar to the following:0.684CH_(1.85)+O₂→0.684CO₂+0.632H₂O  (1)0.316CH_(1.85)+0.316H₂O→0.316CO+0.608H₂  (2)0.316CO+0.316H₂O→0.316CO₂+0.316H₂  (3)wherein CH_(1.85) represents an exemplary reductant, such as dieselfuel, with a 1.85 ratio between carbon and hydrogen. Equation (1) isexothermic complete combustion by which oxygen is consumed. Equation (2)is endothermic steam reforming. Equation (3) is the water gas shiftreaction, which is comparatively thermal neutral and is not of greatimportance in the present disclosure, as both CO and H₂ are effectivefor regeneration.

In an ideal situation, the balance of Equations (1) and (2) providesjust enough heat to maintain the reformer temperature. When the oxygenconcentration is relatively high, e.g., 5-10% or more, depending on thereformer, there may be a tendency for reaction (1) to dominate wherebythe reformer temperature increases to an undesirable degree. Eventually,the reformer 12 must be shutdown to prevent over heating. Once thereformer 12 has cooled, fuel injection can be restarted. The result isthat the reformer controller 106 often causes the fuel supply rate to bepulsed in order to maintain the reformer temperature within anacceptable range, particularly when the exhaust oxygen concentration iscomparatively high.

FIG. 4 illustrates a fuel supply pattern that results from an exemplaryimplementation of the above-described control strategy. Line 401 is aplot of total fuel injected—a quantity that would be monotonicallyincreasing except that it is periodically reset to zero to allowplotting at a scale at which details of line 401 are comparativelyvisible. The fuel injection is relatively periodic on a longer timescale, being characterized by phases in which the total fuel injectionincreases in a staircase fashion and phases in which there is no fuelinjection. During the phases where fuel is injected, the staircasepattern is indicative of pulsed fuel injection on a shorter time scale.

Line 402 of FIG. 4 is a plot of reformer temperatures resulting from thefuel injection pattern of line 401. Line 403 is a pot of resulting LNTtemperatures. Pulsing on the longer time scale can control the LNTtemperatures to within a target range for desulfating the LNT 13.Pulsing on the shorter time scale can control the reformer temperatureto within a target temperature range for efficiently producing reformatewithout overheating the reformer 12. The lengths of the phases orperiods of reductant injection or no reductant injection on the longertime scale are long compared to the corresponding phases or periods onthe shorter time scale.

Accordingly, one of the inventors' concepts is supplying reductant to afirst device upstream of an LNT at a rate that is intermittent on twotime scales, one shorter and one longer. In this context, pulsing orintermittency should be understood as something that can be easilydetected by a hypothetical measurement of the reductant concentration inthe exhaust entering the first device and does not include very rapidpulsing that results in an essentially continuous supply of reductant tothe first device. Specifically, pulsing in the present context does notinclude rapidly switching a valve between fully open and fully closedpositions in order to control the flow rate through the valve.

The pulse periods on the shorter time scale are typically from about0.05 to about 2 seconds, more typically about 0.1 to about 1 second. Thepulse periods on the longer time scale are typically from about 3 toabout 30 seconds. The pulse periods on the longer time scale aretypically at least about 4 times longer than the pulse periods on theshorter time scale, more typically at least about 8 times as long, andstill more typical at least about 12 times as long. These ratios alsotypically apply to the individual phases, e.g., the periods of no fuelinjection on the longer time scale are typically at least about 4 timeslonger than the periods of no fuel injection on the shorter time scale.

As highlighted by the line A-A′ in FIG. 4, the temperature in the LNT 13continues to rise long (at least about 5 seconds in this example) afterfuel injection ceases. While details are uncertain, the temperature riseis related to chemical reactions within the LNT 13. Heat transfer fromthe reformer 12 is not a sufficient explanation in that the temperaturesto which the LNT 13 is rising are above those occurring in the reformer12. While the inventors' concepts are not, in general, tied to anyparticular theory, it is believed that the continued temperature rise isat least in part the result of combustion of adsorbed reductant in theLNT 13, most likely hydrocarbons slipping from the reformer 12.

FIG. 5 illustrates a temporal relationship between temperatures andreductant concentrations within the LNT 13 that result from the fuelinjection plotted in FIG. 4. Line 501 is a plot of hydrogenconcentrations and line 502 is a plot of carbon monoxide concentrations.As highlighted by the line B-B′, temperature peaks within the LNT 13occur significantly later in time than the reformate production peaks.FIG. 5 is meant to illustrate the basis for a concept, rather thanconstitute a preferred embodiment. Preferably the subsequent reformateproduction peaks occurs sooner than shown or the LNT temperature peaksoccur later than shown, whereby there is a coincidence between the LNTtemperature peak and the reformate production peak for the subsequentperiod.

FIG. 6 illustrates the relationship between desulfation rates and theconditions plotted in FIG. 5. Line 601 is a plot of hydrogen sulfideconcentration downstream of the LNT 13. In this example, the greatmajority of the sulfur was released in the form of H₂S, whereby the H₂Sconcentration reflects the desulfation rate. The desulfation rate isshown to be a strong function of temperature, even where the reformateconcentration peaks do not coincide with the LNT temperature peaks. Ifthe reformate concentrations peaks are caused to approach the LNTtemperature peaks, the reformate will be more effectively used.

Another of the inventors' concepts is to time the periods betweenreductant injection phases whereby the peak LNT temperatures occur inthe midst of or near the beginnings of reductant injection phases asopposed to near the ends of the reductant injection phases. The pulseperiods and durations are selected whereby reductant is provided to theLNT 13 while the LNT temperature is still near a peak resulting fromcombustion of reductant injected during the previous pulse. For example,if the LNT temperature varies over each pulse period between a minimumand a maximum differing by at least about 50° C., the LNT temperaturewill generally be within about 25° C. of the maximum at the onsets ofthe reductant injection phases. If the LNT temperature varies over eachpulse period between a minimum and a maximum differing by at least about100° C., the LNT temperature will generally be within about 50° C. ofthe maximum at the onsets of the reductant injection phases.

This concept is most applicable when the temperature continues to risesignificantly after the end of each reductant injection phase on thelonger time scale. Preferably, the LNT temperature continues to rise forat least about 3 seconds after the end of each reductant injectionphase, more preferably at least about 5 seconds. Preferably, the LNTtemperature rises by at least about 40° C. after the end of eachreductant injection phase

FIG. 7 illustrates a control process 700 consistent with the controlarchitecture shown in FIG. 3 and providing an exemplary implementationof several of the above-described concepts. The process 700 begins withoperation 701, determining whether desulfation is required. Thedetermination may be made in any suitable fashion. For example,desulfation may be scheduled periodically, e.g., after every 30 hours ofoperation. Alternatively, the need for desulfation can be determinedbased on system performance, e.g., based on the activity of the LNT 13following an extensive denitration or based on the frequency with whichdenitration is required having increased to an excessive degree.

The denitration process begins with operation 702, warming the reformer12. A typical reformer as contemplated herein is one that operateseffectively only at temperatures above typical diesel engine exhausttemperatures. The reformer 12 can be heated in any suitable fashion. Inthis example, the reformer 12 is heated by injecting fuel at a rate thatkeeps the exhaust at or below a stoichiometric fuel to oxygen ratio.Substantially all the fuel thereby combusts in the reformer 12 toproduce heat and there is essentially no reformate production.

The LNT 13 heats while the reformer 13 is heating, however, after thereformer 12 is fully heated, the LNT may still require further heating.If necessary, at or below stoichiometric operation may be extended toadequately heat the LNT 13. In one example, the LNT 13 is heated to atemperature of at least about 450° C. prior to commencing rich operationwith hydrocarbon slip.

Once the warm-up phase is complete, operation 703 begins. The fuelinjection rate at this stage is generally optimized to give a maximumratio between reformate production and fuel expended. Where thecontroller 10 can throttle the engine air intake or select thetransmission gear ratio, these control parameters can be selected tofacilitate the efficient production and/or usage of the reformate.

Operation 704 determines whether the reformer 12 is overheating.Preferably, the reformer temperature for this purpose is a delay-freeestimate. For example, such an estimate can be made with informationfrom a temperature sensor in the reformer 12 or in the exhaustimmediately downstream of the reformer 12. Forming the estimate caninvolve a simple extrapolation, or can be accomplished using anobserver, such as a Kalman filter or a sliding mode observer. Anobserver preferably comprises a model that takes into accounthydrocarbon adsorption in the reformer 12. It has been observed thathydrocarbon adsorption in the reformer 12 can cause the temperature ofthe reformer 12 to increase significantly after the fuel supply is shutoff. The reformer temperature can also overshoot significantly due tothe delay in receiving temperature measurements unless delay-freeestimates are used.

When the reformer 12 is on the verge of overheating, operation 705 shutsoff the fuel injection. In operation 706, the process 700 waits whilethe reformer 12 cools. The length of the waiting period can bedetermined in any suitable fashion. In one example, operation 706 lastsuntil the reformer 12 has cooled to a target temperature. In anotherexample, there is a fixed period between each short time-scale fuelpulse. In a further example, the length of the period is selecteddynamically by the controller as part of the process of optimizing theamount of reformate production per unit fuel injected. Steps 703-706comprise the inner loop of the control process.

Operation 707 is part of the outer loop. Step 707 determines whether theLNT 13 is getting too hot. Again, a delay-free estimate is preferablyused as the LNT 13 may heat considerably following the termination offuel injection. If the LNT is getting too hot, operation 708 terminatesthe fuel injection. Terminating the fuel injection may comprise issuinginstructions to the inner loop control.

Operation 709 is another waiting operation. In one example, thiscomprises waiting until the LNT 13 has cooled to a target temperature.Preferably, however, there is a fixed period between phases of activefuel injection on the longer time scale. Such a fixed period can be usedto implement the above-described concept of timing these periods inorder that the peak LNT temperature occurs in the midst of or near thebeginning of the phases of active fuel injection.

Following operation 709, the reformer is heated again in operation 702.Operation 702, 707, 708, and 709 comprise the outer loop. If thereformer 12 is of the type that must be heated to operate effectively,heating is generally necessary following a period of no fuel injectionon the longer time scale. The periods of no fuel injection on theshorter time scale are normally selected to avoid having to reheat thereformer 12. After the longer periods of no fuel injection on the longertime scale, the reformer 12 is generally too cool to effectively producereformate without a heating period.

While the engine 9 is preferably a compression ignition diesel engine,the various concepts of the invention are applicable to power generationsystems with lean-burn gasoline engines or any other type of engine thatproduces an oxygen rich, NOx-containing exhaust. For purposes of thepresent disclosure, NOx consists of NO and NO₂.

The transmission 8 can be any suitable type of automatic transmission.The transmission 8 can be a conventional transmission such as acounter-shaft type mechanical transmission, but is preferably a CVT. ACVT can provide a much larger selection of operating points than aconventional transmission and generally also provides a broader range oftorque multipliers. In general, a CVT will also avoid or minimizeinterruptions in power transmission during shifting. Examples of CVTsystems include hydrostatic transmissions; rolling contact tractiondrives; overrunning clutch designs; electrics; multispeed gear boxeswith slipping clutches; and V-belt traction drives. A CVT may involvepower splitting and may also include a multi-step transmission.

A preferred CVT provides a wide range of torque multiplication ratios,reduces the need for shifting in comparison to a conventionaltransmission, and subjects the CVT to only a fraction of the peak torquelevels produced by the engine. This can be achieved using a step-downgear set to reduce the torque passing through the CVT. Torque from theCVT passes through a step-up gear set that restores the torque. The CVTis further protected by splitting the torque from the engine, andrecombining the torque in a planetary gear set. The planetary gear setmixes or combines a direct torque element transmitted from the enginethrough a stepped automatic transmission with a torque element from aCVT, such as a band-type CVT. The combination provides an overall CVT inwhich only a portion of the torque passes through the band-type CVT.

The fuel injector 11 can be of any suitable type. Preferably, itprovides the fuel in an atomized or vaporized spray. The fuel may beinjected at the pressure provided by a fuel pump for the engine 9.Preferably, however, the fuel passes through a pressure intensifieroperating on hydraulic principles to at least double the fuel pressurefrom that provided by the fuel pump to provide the fuel at a pressure ofat least about 4 bar.

The lean-NOx catalyst 15 can be either an HC-SCR catalyst, a CO-SCRcatalyst, or a H₂-SCR catalyst. Examples of HC-SCR catalysts includetransitional metals loaded on refractory oxides or exchanged intozeolites. Examples of transition metals include copper, chromium, iron,cobalt, nickel, cadmium, silver, gold, iridium, platinum and manganese,and mixtures thereof. Exemplary of refractory oxides include alumina,zirconia, silica-alumina, and titania. Useful zeolites include ZSM-5, Yzeolites, Mordenite, and Ferrerite. Preferred zeolites have Si:Al ratiosgreater than about 5, optionally greater than about 20. Specificexamples of zeolite-based HC-SCR catalysts include Cu-ZSM-5, Fe-ZSM-5,and Co-ZSM-5. A CeO₂ coating may reduce water and SO₂ deactivation ofthese catalysts. Cu/ZSM-5 is effective in the temperature range fromabout 300 to about 450° C. Specific examples of refractory oxide-basedcatalysts include alumina-supported silver. Two or more catalysts can becombined to extend the effective temperature window.

Where a hydrocarbon-storing function is desired, zeolites can beeffective. U.S. Pat. No. 6,202,407 describes HC-SCR catalysts that havea hydrocarbon storing function. The catalysts are amphoteric metaloxides. The metal oxides are amphoteric in the sense of showingreactivity with both acids and bases. Specific examples includegamma-alumina, Ga₂O₃, and ZrO₂. Precious metals are optional. Whereprecious metals are used, the less expensive precious metals such as Cu,Ni, or Sn can be used instead of Pt, Pd, or Rh.

In the present disclosure, the term hydrocarbon is inclusive of allspecies consisting essentially of hydrogen and carbon atoms, however, aHC-SCR catalyst does not need to show activity with respect to everyhydrocarbon molecule. For example, some HC-SCR catalysts will be betteradapted to utilizing short-chain hydrocarbons and HC-SCR catalysts ingeneral are not expected to show substantial activity with respect toCH₄.

Examples of CO-SCR catalysts include precious metals on refractory oxidesupports. Specific examples include Rh on a CeO₂—ZrO₂ support and Cuand/or Fe ZrO₂ support.

Examples of H₂-SCR catalysts also include precious metals on refractoryoxide supports. Specific examples include Pt supported on mixed LaMnO₃,CeO₂, and MnO₂, Pt supported on mixed ZiO₂ and TiO₂, Ru supported onMgO, and Ru supported on Al₂O₃.

The lean-NOx catalyst 15 can be positioned differently from illustratedin FIG. 1. In one embodiment, the lean NOx catalyst 15 is upstream ofthe fuel injector 11. In another embodiment the lean NOx catalyst isdownstream of the reformer 12, whereby the lean NOx catalyst 15 can usereformer products as reductants. In a further embodiment, the lean NOxcatalyst 15 is well downstream of the LNT 13, whereby the lean NOxcatalyst 15 can be protected from high temperatures associated withdesulfating the LNT 13.

A fuel reformer is a device that converts heavier fuels into lightercompounds without fully combusting the fuel. A fuel reformer can be acatalytic reformer or a plasma reformer. Preferably, the reformer 12 isa partial oxidation catalytic reformer. A partial oxidation catalyticreformer comprises a reformer catalyst. Examples of reformer catalystsinclude precious metals, such as Pt, Pd, or Ru, and oxides of Al, Mg,and Ni, the later group being typically combined with one or more ofCaO, K₂O, and a rare earth metal such as Ce to increase activity. Areformer is preferably small in size as compared to an oxidationcatalyst or a three-way catalyst designed to perform its primaryfunctions at temperatures below 500° C. A partial oxidation catalyticreformer is generally operative at temperatures from about 600 to about1100° C.

The NOx absorber-catalyst 13 can comprise any suitable NOx-adsorbingmaterial. Examples of NOx adsorbing materials include oxides,carbonates, and hydroxides of alkaline earth metals such as Mg, Ca, Sr,and Be or alkali metals such as K or Ce. Further examples ofNOx-adsorbing materials include molecular sieves, such as zeolites,alumina, silica, and activated carbon. Still further examples includemetal phosphates, such as phosphates of titanium and zirconium.Generally, the NOx-adsorbing material is an alkaline earth oxide. Theabsorbent is typically combined with a binder and either formed into aself-supporting structure or applied as a coating over an inertsubstrate.

The LNT 13 also comprises a catalyst for the reduction of NOx in areducing environment. The catalyst can be, for example, one or moreprecious metals, such as Au, Ag, and Cu, group VIII metals, such as Pt,Pd, Ru, Ni, and Co, Cr, Mo, or K. A typical catalyst includes Pt and Rh,although it may be desirable to reduce or eliminate the Rh to favor theproduction of NH₃ over N₂. Precious metal catalysts also facilitate theabsorbent function of alkaline earth oxide absorbers.

Absorbents and catalysts according to the present invention aregenerally adapted for use in vehicle exhaust systems. Vehicle exhaustsystems create restriction on weight, dimensions, and durability. Forexample, a NOx absorbent bed for a vehicle exhaust systems must bereasonably resistant to degradation under the vibrations encounteredduring vehicle operation.

An absorbent bed or catalyst brick can have any suitable structure.Examples of suitable structures may include monoliths, packed beds, andlayered screening. A packed bed is preferably formed into a cohesivemass by sintering the particles or adhering them with a binder. When thebed has an absorbent function, preferably any thick walls, largeparticles, or thick coatings have a macro-porous structure facilitatingaccess to micro-pores where adsorption occurs. A macro-porous structurecan be developed by forming the walls, particles, or coatings from smallparticles of adsorbant sintered together or held together with a binder.

The ammonia-SCR catalyst 14 is a catalyst effective to catalyzereactions between NOx and NH₃ to reduce NOx to N₂ in lean exhaust.Examples of SCR catalysts include oxides of metals such as Cu, Zn, V,Cr, Al, Ti, Mn, Co, Fe, Ni, Pd, Pt, Rh, Rd, Mo, W, and Ce, zeolites,such as ZSM-5 or ZSM-11, substituted with metal ions such as cations ofCu, Co, Ag, Zn, or Pt, and activated carbon. Preferably, the ammonia-SCRcatalyst 14 is designed to tolerate temperatures required to desulfatethe LNT 13.

The particulate filter 16 can have any suitable structure. Examples ofsuitable structures include monolithic wall flow filters, which aretypically made from ceramics, especially cordierite or SiC, blocks ofceramic foams, monolith-like structures of porous sintered metals ormetal-foams, and wound, knit, or braided structures of temperatureresistant fibers, such as ceramic or metallic fibers. Typical pore sizesfor the filter elements are about 10 μm or less. Optionally, one or moreof the reformer 12, the LNT 13, the lean-NOx catalyst 15, or theammonia-SCR catalyst 14 is integrated as a coating or within thestructure of the DPF 16.

The DPF 16 is regenerated to remove accumulated soot. The DPF 16 can beof the type that is regenerated continuously or intermittently. Forintermittent regeneration, the DPF 16 is heated, using a reformer 12 forexample. The DPF 16 is heated to a temperature at which accumulated sootcombusts with O₂. This temperature can be lowered by providing the DPF16 with a suitable catalyst. After the DPF 16 is heated, soot iscombusted in an oxygen rich environment.

For continuous regeneration, the DPF 16 may be provided with a catalystthat promotes combustion of soot by both NO₂ and O₂. Examples ofcatalysts that promote the oxidation of soot by both NO₂ and O₂ includeoxides of Ce, Zr, La, Y, and Nd. To completely eliminate the need forintermittent regeneration, it may be necessary to provide an additionaloxidation catalyst to promote the oxidation of NO to NO₂ and therebyprovide sufficient NO₂ to combust soot as quickly as it accumulates.Where regeneration is continuous, the DPF 16 is suitably placed upstreamof the reformer 12. Where the DPF 16 is not continuously regenerated, itis generally positioned as illustrated downstream of the reformer 12. Anadvantage of the position illustrated in FIG. 2 is that the DPF 16buffers the temperature between the reformer 12 and the LNT 13.

The clean-up catalyst 17 is preferably functional to oxidize unburnedhydrocarbons from the engine 9, unused reductants, and any H₂S releasedfrom the NOx absorber-catalyst 13 and not oxidized by the ammonia-SCRcatalyst 15. Any suitable oxidation catalyst can be used. A typicaloxidation catalyst is a precious metal, such as platinum. To allow theclean-up catalyst 17 to function under rich conditions, the catalyst mayinclude an oxygen-storing component, such as ceria. Removal of H₂S,where required, may be facilitated by one or more additional componentssuch as NiO, Fe₂O₃, MnO₂, CoO, and CrO₂.

The invention as delineated by the following claims has been shownand/or described in terms of certain concepts, components, and features.While a particular component or feature may have been disclosed hereinwith respect to only one of several concepts or examples or in bothbroad and narrow terms, the components or features in their broad ornarrow conceptions may be combined with one or more other components orfeatures in their broad or narrow conceptions wherein such a combinationwould be recognized as logical by one of ordinary skill in the art.Also, this one specification may describe more than one invention andthe following claims do not necessarily encompass every concept, aspect,embodiment, or example described herein.

1. A method of operating a power generation system, comprising:operating a compression ignition diesel engine to produce a leanexhaust; channeling the exhaust through a fuel reformer and a leanNO_(X) trap, in that order; from time-to-time, heating the fuel reformerto an operating temperature range and the lean NO_(X) trap to adesulfating temperature range; and maintaining the lean NO_(X) trapwithin the desulfating temperature range while desulfating the leanNO_(X) trap by controlling a fuel injection into the lean exhaustupstream from the fuel reformer to be intermittent, creating alternatinglean and rich phases within the fuel reformer and the lean NO_(X) trap,in a manner that causes fuel, or reformed fuel from the rich phases toreact with oxygen from the lean phases within the lean NO_(X) trap to anextent that contributes to maintaining the lean NO_(X) trap within thedesulfating temperature range, causes the lean NO_(X) trap to reachtemperatures exceeding those reached by the fuel reformer, and enablesthe lean NO_(X) trap to reach a peak temperature that is at least 50° C.above the peak temperature of the exhaust entering the lean NO_(X) trap;wherein during the rich phases, the fuel reformer consumes the majorityof the oxygen in the exhaust while producing reformate; and the leanNO_(X) trap is a device that adsorbs NO_(X) under lean conditions andreduces and releases adsorbed NO_(X) under rich conditions.
 2. Themethod of claim 1, wherein controlling the fuel injection to maintainthe lean NO_(X) trap within the desulfating temperature range comprisescontrolling the periods and durations of the lean and rich phases. 3.The method of claim 1, further comprising controlling the fuel reformertemperature by controlling the fuel injection.
 4. The method of claim 3,wherein the fuel injection control controls the temperature of the fuelreformer independently from the temperature of the lean NO_(X) trap. 5.The method of claim 1, wherein the reactions within the lean NO_(X) trapcause the lean NO_(X) trap to reach a peak temperature that is at least100° C. above the peak temperature of the exhaust entering the lean NOXtrap.
 6. The method of claim 1, wherein the rich conditions are notproduced unless the fuel reformer is within its operating temperaturerange, which is a range above 450° C.
 7. The method of claim 6, furthercomprising following lean phases in which the fuel reformer cools belowits operating temperature range, injecting fuel into the exhaust tocreate combustion within the fuel reformer under lean conditions to heatthe fuel reformer to within its operating temperature range.
 8. A powergeneration system comprising: an engine; a fuel reformer; a lean NO_(X)trap; and a controller; wherein the controller is programmed to operateaccording to the method of claim
 1. 9. A vehicle comprising the powergeneration system of claim 8.